Filtration treatment using filter membranes is employed as a method for removing polluting substances from water to be treated. For example, in the case of sewerage, biological treatment is performed by an activated sludge process and the like, and then polluting substances such as organic substances are separated and removed using filter membranes. Cylindrical microfiltration membranes or ultrafiltration membranes are generally used as filter membranes. Moreover, examples of filtration modes include an external pressure filtration mode that involves causing water to be treated to flow outside the cylindrical filter membrane and causing filtrate water to flow inside same, an internal pressure filtration mode that involves causing water to be treated to flow inside the cylindrical filter membrane and causing filtered water to flow outside same. Filtration treatment in which such a cylindrical filter membrane is used is problematic in that as the filter membrane is continuously used, filtration performance decreases. Specifically, as a filter membrane is continuously used, polluting substances adhere to the surface (outer surface in the external pressure filtration mode, and inner surface in the internal pressure filtration mode) of the filter membrane that comes into contact with water to be treated, the surface (inner surface in the external pressure filtration mode, and outer surface in the internal pressure filtration mode) of the filter membrane or into the pores of the membrane that comes into contact with filtered water, thereby causing fouling and a gradual decrease in filtration performance. In particular, when fouling takes place on a filter membrane, the pressure required at the time of filtration increases, and thus the membrane filtration flux (the amount of water that passes through the filtration membrane per unit of time and per unit area of membrane) also decreases. Therefore, periodic cleaning of a filter membrane is required to maintain its performance.
Hence, as a method for maintaining filtration performance, backflow cleaning is performed, by which filtered water and cleaning water such as clear water are passed from the filtered water side opposite to the filtration direction of the filter membrane through the filter membrane, so as to physically remove polluting substances adhered to the surface of the filter membrane that comes into contact with water to be treated. Moreover, to increase the effect of removing polluting substances, a method for backflow cleaning using cleaning water containing an oxidizing agent such as sodium hypochlorite and ozone, in which the polluting substances that are chemically adhered with intermolecular forces to the surface of a filter membrane or into the pores of the membrane contacting with filtered water are oxidatively degraded and removed, has been proposed (for example, see Patent Documents 1 and 2). Another method that has been proposed involves oxidatively degrading polluting substances chemically adhered to the surface of a filter membrane or into the pores of the membrane that comes into contact with filtered water using cleaning water containing an oxidizing agent, and then blowing air containing ozone over the surface of the filter membrane that comes into contact with water to be treated, thereby removing polluting substances adhered to the filter membrane (for example, see Patent Document 3). Furthermore, another method that has been proposed involves contacting hydrogen peroxide with ozone within a filter membrane, generating a radical such as a hydroxyl radical with oxidizability higher than that of ozone, to thus remove polluting substances adhered to the surface of the filter membrane that comes into contact with water to be treated, the surface of the filter membrane or into the pores of the membrane that comes into contact with filtered water via oxidative degradation (for example, see Patent Document 4).